The present invention relates to a display apparatus in which a large multi-screen is constituted by a plurality of display units such as projection TVs. More particularly, the invention is directed to an ABL (Automatic Brightness Limiter) circuit which automatically adjusts brightness or contrast of a picture displayed on each display unit.
As shown in FIG. 4, in a multi-vision-type display apparatus, a large screen is formed by a plurality of display units U1-U9 arranged, e.g., in matrix form, which receive related video signals, whereby an enlarged single picture is displayed over the entire screen or different pictures are displayed on the respective display units U1-U9.
Each of the display units U1-U9 has projectors for emitting three light beams corresponding to respective color video signals of R, G and B. The three light beams are projected onto a projection screen using, e.g., a Fresnel lens, to reproduce a picture. To adjust the brightness of the screen, an ABL circuit is provided in each of the display units U1-U9.
However, when a large screen is formed by combining a plurality of display units U1-U9 in the above manner, there arises such a problem that the screen brightness varies from one display unit to another. Assume now a case in which a picture as shown in FIG. 4 is displayed on the display units U1-U9; that is, a major white portion W and a small black portion B are displayed on the display unit U9. Since a conventional video display circuit performs the ABL operation on a single unit basis, the ABL operation will be actuated in the display unit U9 having the large white portion W, thereby causing the black portion B to be further darkened. Since the brightness limitation is not actually effected in the display units U1-U8, no darkening such as in the black portion B of the display unit U9 will not occur. Thus, there is inconsistency in the display of the same black color between the display units U1-U8 and the display unit U9 and, as a result, the displayed picture on the multi-screen becomes unnatural.
To overcome this inconvenience, there has been developed a video display circuit in which a common ABL signal is applied to respective display units U1-U9 to equalize their brightness (Japanese Utility Model Application Unexamined Publication No. Sho. 63-55683).
However, since this conventional video display circuit operates so as to forcibly provide the common ABL signal, there is a possibility that the screens of some display units become too dark. This is explained more specifically. The minimum ABL voltage among the ABL voltages of the display units U1-U9 is employed as the commen ABL signal. Therefore, while the brightness limitation is properly effected in the display units having ABL voltages close to the minimum ABL voltage, the brightness remains dark in the display units having higher ABL voltages.